chang’s attenuation correction method hossein aslian
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A general overview of Chang’s attenuation correction methodTRANSCRIPT
“Chang attenuation correction method on SPECT”
Final PresentationNuclear medicine2Hossein AslianJuly14-2014
Contents:1.Introduction2.The definition and purpose3.Chang method4.Conclusion5.References
1
Rotating scintillator camera
In SPECT similar to CT-scan, projection data are acquired from different view around patient.
Although image processing and image reconstruction methods are the same as CT, there are some significant differences between them.
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1. Introduction
One of the most significant differences is that SPECT images depredate due to:
Photon Attenuation
Scattering
Detector Response
Importing and registering maps from
another modality
Images are reconstructed regardless of photons attenuation
amount
Image reconstruction is done through attenuation simplification
They are three methods for attenuationcorrection from the perspective of image
reconstruction
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1. Introduction
Degradation caused by attenuation!
Intensity is higher due to lower attenuation
Intensity is lower due to higher attenuation
Consequence of attenuation
Where in the body radioactivity is locatedDepends on size of the patient and tissueDepends on energy of radiation activity
Therefore we have to reduce the effect of Therefore we have to reduce the effect of attenuationattenuation
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2. Definition and purpose
Fig.1: Fig.1: H.Ziadi,”Determination H.Ziadi,”Determination of the Attenuation Map of the Attenuation Map in Emission in Emission Tomography”,J Nucl Med Tomography”,J Nucl Med , 2003, 2003
From Prof.Bertocchi lecture notes we know that a general way to compensate attenuation is conjugate counting
It refers to acquiring data for directly opposing views and then combining these data into a single data set.
Detector.1
Detector.2
Dba
1) Arithmetic mean
2) Geometric mean
So, it depends on total tissue thickness D, but not on source depth So, it depends on total tissue thickness D, but not on source depth a,ba,b
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3. Attenuation correctionBasic ways
Chang attenuation correction method Even with geometric mean we cannot have a quantitative accuracy.The best-known and most widely employed method thatperforms AC with FBP is due to Chang (1978).
FBPIt is the most commonly used
method for image reconstruction in SPECT
However physical phenomena cannot be modeled by FBP:
In particular effect of attenuation
Chang proposed one post reconstruction
method
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3. Chang Method
1. An initial image reconstructed by FBP 2. Estimated object geometry and counter to measure path length
3. Assumed the linear attenuation at given energy is constant for all body tissue:4. Using Attenuation Correction Factor for each pixel
5. Multiplying the reconstructed image by ACF for each pixel and finding corrected image
Chang attenuation correction Steps:
=
6. Next slides
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3. Chang Method
Chang ACF term:
Proj
ectio
n
Pixel(x1,y1)
d1
d2Pixel(x2,y2)
M : number of projectionMu : attenuation coefficientdi : path length
d1>d2: ACF1>ACF2
Gray level pixel1< Gray level pixel2
AFC1 pixel1=more increase AFC1 pixel1=less increase
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3. Chang Method
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3. Chang Method
Fig.2
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3
Schematics of Profiles
Pixel number
Pixel number
Pixel number
Pix
el V
alu
e
Pix
el V
alu
e
Pix
el V
alu
e
Complete implementation of chang method:Step 6: Finding error projection
6.1: forward projected profile obtained:Forward projection is the image obtained with the first order correction
6.2 subtract the forward projected from the original measured projection profile
Final attenuation corrected image by chang Final attenuation corrected image by chang methodmethod
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3. Chang Method
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4. Conclusion
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4.2 Attenuation correction methods
1.Ch.Chen, "Emission Tomography",Elsevier press,2004
2.A.Webb, "Introduction into biomedical imaging system",Wiely,2005
3. Jennifer Prekeges, "Nuclear Medicine Instrumentation", Jones & Bartlett Learning,2009
4. Lecture notes by Professor Bertochi and professor Benuti,MMP2014,ICTP
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5. References
Full of thanks
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